This invention relates to patch antennas, particularly (but not exclusively) for microwave or near-microwave applications. The invention has particular utility in the provision of antennas for attachment to, or formed on or built into, the windows of buildings, or the windows or nonmetallic body panels of vehicles.
For convenience, the invention is described and claimed in terms of a transmitting antenna. However, since an antenna is intrinsically a functionally reversible device, the invention relates equally to antennas for use in a transmitting mode, a receiving mode, or both, and the claims are to be interpreted as extending to all such antennas;
FIG. 1 of the accompanying drawings shows a conventional patch antenna, consisting of a conducting radiating element or xe2x80x9cpatchxe2x80x9d 10 separated from a conducting ground plane 12 by a dielectric layer 14. The patch is a piece of foil or other sheet-like conductive material adhered or printed on to the surface of the dielectric, and is of continuous and uniform section (thickness) in its plane. It will be appreciated that the term xe2x80x9cplanexe2x80x9d is here used figuratively; the antenna, if conforming to a curved surface, such as a body panel or rear window of a vehicle, will not be strictly planar in the geometrical sense.
The radiating element is fed either by a physical connection to a suitable conductor or is electromagnetically coupled to a microstrip line.
In such antennas transmission is effected via fringing fields between the radiating element 10 and the ground plane 12. These fields depend on the distribution of the currents flowing in the radiating element and the ground plane.
The current distribution is in accordance with one of several modes which are determined by the shape and size of the patch. It is not possible to configure the antenna always to operate in the desired mode across its full operating range, with the result that the radiation pattern of the antenna is adversely affected. The performance of the antenna, in particular bandwidth and gain for example, then may be reduced compared to its performance in the preferred mode.
European Patent Application No. 0 805 512 A1 (Brachat) discloses an antenna having a circular radiating element with four radial slots, and also comprising a series of diodes each bridging a respective one of the slots. A generally circular current pattern is present in each of the quadrants of the radiating element when a signal is transmitted or received on the radiating element.
The present invention, at least in its preferred embodiments, is directed to alleviating this problem. Some preferred embodiments also provide an antenna with a useful degree of transparency which can be attached to or incorporated in a vehicle windscreen or other window.
The invention provides in one aspect a patch antenna comprising a radiating element and a ground plane with a dielectic therebetween, one or both of the radiating element and the ground plane comprising a plurality of spaced discrete first conductors each defining a respective one of a series of spaced discrete elongated conductors whose longitudinal extents form a pattern approximating to the pattern of a preferred excitation mode in an equivalent patch radiating element of uniform and continuous cross-section.
A second aspect of the invention relates to a window glass or other optically-transparent substrate or a vehicle body part that comprises the patch antenna of the invention.
A third aspect of the invention provides a method of transmitting or receiving radio-frequency energy utilizing the patch antenna of the invention.
Preferably, one or both of the radiating element and ground plane also comprise means for permitting minor currents to flow between respective adjacent pairs of the elongated conductors and, more preferably, that means comprises a plurality of spaced discrete second conductors that intersect the elongated conductors to form a grid pattern. Still more preferably, the plurality of first conductors intersect the plurality of second conductors substantially orthogonally.
The antenna may means for stimulating circularly-polarised emission from the radiating element, and the pattern of the preferred excitation mode may approximate to au excitation mode of a circular or annular patch. Thus, it may be the TM01, TM11, TM21, TM10, or TM12 mode of such a patch. Other possible modes (without limitation) are TM31 and TM41.
Alternatively, the pattern may occupy a substantially rectangular or square shape.
Thus (without limitation) the pattern may approximate to the TM10, TM11, TM21, TM01 or TM12 mode for a uniform square or rectangular patch. Alternatively, the conductors of the square or rectangular pattern may be substantially parallel to a diagonal thereof.
Means may be provided for stimulating circularly-polarised emission from the radiating element.
Thus the radiating element may comprise at least one notch or projection whereby it is adapted to emit circularly-polarised signals.
Opposite corners of a square shape pattern may be absent whereby the radiating element is adapted to emit circularly-polarised signals.
Preferably, there is a solid dielectric disposed between the radiating element and the ground plane.
In one preferred form of the invention, the radiating element and the ground plane both comprise a said conductor pattern, and the dielectric is transparent. Preferably the patterns of the radiating element and the ground plane are identical.
One or both of radiating element and the ground plane may be printed on or otherwise carried by a glass or other transparent substrate.
The invention also comprises a window glass or other optically transparent substrate or a vehicle body part comprising an antenna as set forth above.